Previous work done has shown that the cytosolic factor which inhibits lipid peroxidation is not glutathione peroxidase and this has been confirmed by others. Our investigations have shown that a glutathione-dependent, heat-labile factor, previously thought to be glutathione peroxidase, inhibits lipid peroxidation by preventing loss of polyunsaturated fatty acids (PUFA) in biological membranes under conditions that would otherwise promote a high rate of PUFA degradation. The system inhibits both non-enzymic and enzymic lipid peroxidation, including that which accompanies CCl4 metabolism. The factor is present in liver cytosol and mitochondria but whether they are identical in nature is not known. We have also shown it is present in rat heart and lung and in guinea pig liver. It has been proposed that the factor may be one of more of the glutathione-S-transferases, but our most recent studies indicate that it is not a transferase. The system appears to be one of the most potent protective mechanisms against oxidative stress in those tissues. We propose to isolate the factor, to study its properties, and to investigate the mechanism by which the system prevents peroxidation of membrane lipids. We propose to determine if the system is a constitutive component of animal tissues, and to determine 1) the possible role of membrane -SH, -S-S-, and mixed disulfide groups in the functioning of the system; 2) whether or not the GSH-dependent factor is functioning as an efficient radical scavenging system; and 3) the status of this system in animals, as a function of age. Certain tissues of young animals are very susceptible to oxidative stress while old animals show accumulations of ceroid pigments in some tissues, particularly if tocopherol-deficient. We propose to investigate the capacity of this system to function in relation to animal age and under certain oxidative stresses. Conditions which may decrease the efficiency of this system could be expected to have serious consequences in cells. Examples of the latter are discussed in the Significance.